Re: [lttng-dev] Capturing User-Level Function Calls/Returns

[ahmadkhorrami via lttng-dev <lttng-dev@lists.lttng.org>]

So, the only barrier to the user-level implementation is the problem 
with instruction sizes. That's an enlightening point. Thanks for the 
detailed answer!
Thanks everybody specially Steven and Mathieu.

Regards.

On 2020-07-16 02:18, Steven Rostedt wrote:

> On Thu, 16 Jul 2020 02:09:50 +0430
> ahmadkhorrami <ahmadkhorrami@ut.ac.ir> wrote:
> 
>> Hi Steven and Mathieu,
>> Firstly, many thanks! This method seems to be the most efficient 
>> method.
>> But, IIUC, what you suggest requires source code compilation. I need 
>> an
>> efficient dynamic method that, given the function address, captures 
>> its
>> occurrence and stores some information from the execution context. Is
>> there anything better than Uprobes perhaps with no trap into the 
>> kernel?
>> Why do we need traps?
>> Regards.
> 
> Without recompiling, how would that be implemented?
> 
> You would need to insert a jump on top of code, and still be able to
> preserve that code. What a trap does, is to insert a int3, that will
> trap into the kernel, it would then emulate the code that the int3 was
> on, and also call some code that can trace the current state.
> 
> To do it in user land, you would need to find way to replace the code
> at the location you want to trace, with a jump to the tracing
> infrastructure, that will also be able to emulate the code that the
> jump was inserted on top of. As on x86, that jump will need to be 5
> bytes long (covering 5 bytes of text to emulate), where as a int3 is a
> single byte.
> 
> Thus, you either recompile and insert nops where you want to place your
> jumps, or you trap using int3 that can do the work from within the
> kernel.
> 
> -- Steve
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